Improve gas efficiency in ECDSA

CHANGELOG.md

@@ -6,6 +6,7 @@
  * `ERC20Votes`: optimize by using unchecked arithmetic. ([#3748](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3748))
  * `Initializable`: optimize `_disableInitializers` by using `!=` instead of `<`. ([#3787](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3787))
  * `Math`: optimize `log256` rounding check. ([#3745](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3745))
+ * `ECDSA`: optimize bytes32 computation by using assembly instead of `abi.encodePacked`. ([#3853](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3853))
 
 ### Deprecations
 

contracts/utils/cryptography/ECDSA.sol

@@ -180,10 +180,14 @@ library ECDSA {
      *
      * See {recover}.
      */
-    function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32) {
-        // 32 is the length in bytes of hash,
+    function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32 message) {
         // enforced by the type signature above
-        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
+        /// @solidity memory-safe-assembly
+        assembly {
+            mstore(0, "\x19Ethereum Signed Message:\n32")
+            mstore(28, hash)
+            message := keccak256(0, 60)
+        }
     }
 
     /**
@@ -207,7 +211,14 @@ library ECDSA {
      *
      * See {recover}.
      */
-    function toTypedDataHash(bytes32 domainSeparator, bytes32 structHash) internal pure returns (bytes32) {
-        return keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
+    function toTypedDataHash(bytes32 domainSeparator, bytes32 structHash) internal pure returns (bytes32 data) {
+        /// @solidity memory-safe-assembly
+        assembly {
+            let ptr := mload(0x40)
+            mstore(ptr, "\x19\x01")
+            mstore(add(ptr, 0x02), domainSeparator)
+            mstore(add(ptr, 0x22), structHash)
+            data := keccak256(ptr, 66)
+        }
     }
 }